The accuracy in which liquids can be dispensed influences the precision of scientific instruments and manual operations. Devices for dispensing liquids include pipettes, capillary tubes, droppers, air and positive displacement pipettes and syringes. These devices can be operated manually or can be incorporated into larger instrumentation.
Hydrophobicity plays an important role in the accuracy of liquid dispensing devices. As used herein the term "hydrophobic" is used in the sense of repelling water. The term "hydrophilic" is used in an opposite sense, namely, attracting water.
One source of error arising in devices which dispense liquids is the adherence of liquid to the housing of such device. In the case of a pipette, fluid may cling to the inside or outside of the tip. Such fluid does not release uniformly from the orifice, altering the amount of fluid dispensed.
In using pipettes, two operations are often performed. First, the tip of the pipette is inserted into liquid and liquid is drawn in to the body cavity of the pipette through an opening or orifice. Second, the pipette is positioned over a second vessel and a measured amount of liquid released from the cavity. Liquid clinging to the outside of the tip from a first sample, can be transferred, as part of the second operation, into a next sample causing an error of too much liquid being dispensed. In the alternative, liquid which is intended to be released into a sample may cling to the tip causing too little fluid being dispensed.
Pipettes and pipette tips have been formed of plastics, including polystyrene. Although such plastics have been generally described as being hydrophobic, during the process of manufacturing, such hydrophobicity is often altered. For example, in rendering liquid dispensing devices sterile, such devices are generally irradiated. Irradiation generates charged groups and free radicals on the surface of the plastics and within the surrounding atmosphere. Gaseous molecules within the atmosphere such as ozone or free radical oxygen are highly reactive. Such free radicals and charged groups on the surface of the polystyrene, polyethlyene or polypropylene in the presence of the atmosphere, react in a manner which functionalizes the surface, altering its hydrophobicity.
In addition, in molding processes used in making devices of polystyrene, polyethlyene and polypropylene, lubricants are commonly used to facilitate removal of the device from the mold. Common mold lubricants include stearic acid and zinc stearate. Such mold lubricants are also oxidized during the irradiation process and become less hydrophobic.
There are a number of post molding treatments to render polymer surfaces hydrophobic such as silicone oils and various oils of hydrocarbon origin. The disadvantage is that oils migrate and dissolve in various solvents as well as adding contaminants to the fluid being contained within them. These surfaces are inherently unstable and can yield variable results. In addition, irradiation can alter their hydrophobicity and therefore their function.
Fluoropolymer lubricating agents have been used for both exterior and interior lubrication of polyolefin films during extrusion processes, for improving surface finish and for improving the melt rheology characteristics.
There are many articles of manufacture for which retention of liquids or aqueous soluble materials may be important. By way of example, the function of containment vessels; probes, such as temperature or flow sensors; solid supports for capturing molecules of interest; filters; membranes; and tubing for conveying liquids; may be influenced by carry over from one liquid sample to another. Such carry over may be a source of error or contamination.